This invention relates generally to removal of particles from an aerosol, and, more particularly, to an apparatus and method for removing particles without appreciably affecting the thermodynamic properties or chemical composition of the gas phase of the aerosol.
Particles distributed in gas have various effects in the environment, technical applications, and measurement devices. To, for example, enable research investigations on particle and gas measurements, particles have to be removed from the gas phase of an aerosol. So far, mainly fabric filters and in some cases, electrical filters have been employed. However, these known approaches suffer from serious drawbacks in certain applications.
Recently, a differential particulate mass monitor which intrinsically corrects for volatilization losses has been introduced. As described in U.S. Pat. No. 6,205,842 B1, this mass monitor employs alternately activatable particle removers for selectively removing substantially all particulate matter from a gas stream, without appreciably affecting gas stream temperature, pressure and flow rate. This patent (which is hereby incorporated by reference herein in its entirety) teaches that xe2x80x9cSuch particle removal can be advantageously implemented using an electrostatic precipitator of the same general type as is commonly used in air cleaning equipment. In order to reduce ozone production, an electrostatic precipitator operating with a positive corona and very low current, e.g. on the order of tens-hundreds nanoamps, is preferred. The current should be sufficient to cause the precipitator to remove substantially all particulate matter from the gas stream.xe2x80x9d (Column 6, lines 48-56)
Ideally, a particle remover for use in such a differential particulate mass monitor should fulfill the particle separation function without affecting the gas phase thermodynamic conditions or chemical composition.
Fabric filters are available in different sizes, shapes and materials. They are used for a broad variety of applications. Small filters are used for air cleaning to protect measuring instruments and for manual sampling of ambient particles for mass concentration determinations. Large fabric filters are used to clean flue gases from industrial and power plants.
Fabric filters remove particles from a sample gas stream with high efficiency, but the pressure drop across the filter is high and increases with increasing filter loading. Hence, the gas pressure downstream of the filter is lower than the actual ambient gas pressure. Further, the gas phase of the sample is altered due to evaporation of particles at the filter surface. Also, handling of fabric filters in alternating operation is complicated. The filters have to be removed from the gas stream, when ambient particle concentrations are required behind the filter and moved back in-line when particles need to be removed. Frequent maintenance and filter changing are necessary.
In common electrostatic precipitators (ESP""s), particles are charged by a corona discharge. The charged particles are deflected towards a precipitation electrode due to electrostatic forces. The size and geometrical arrangement of ESP""s differ according to application requirements. Common arrangements include (multi) wire-plate (mainly for industrial use, e.g. flue gas treatment and indoor air cleaners), and pin-plate and wire-tube (both mainly for scientific, laboratory scale applications).
Common ESP""s separate gas and particles with a high efficiency. The pressure drop across the ESP is generally low and alternating operation is easy by simply switching the power supply on and off. On the other hand, the gas phase of the sample is changed significantly, mainly due to formation of ozone and nitrogen oxides by the corona discharge. Another process leading to an alteration of the gas composition is evaporation of particles precipitated on the collecting electrode.
Wet ESP""s are usually employed in industrial applications, such as flue gas treatment of industrial and power plants. They operate like common ESP""s, but particles precipitated on the collecting electrode are flushed away by a thin water layer. This treatment prevents particles from agglomerating on the precipitation electrode surface that may form tips. These tips may cause opposite corona discharges leading to particle re-entrainment. Further, the treatment prevents particles on the collecting electrode from evaporating; although the gas phase of the aerosol is still significantly altered due to the formation of ozone and nitrogen oxides from the corona discharge. Additionally, the gas gets humidified by the water. In the differential particulate mass monitor application, for example, humidification of the aerosol could cause several severe problems, including change of the particle phase due to condensation of water on the particle surface and alteration of the particles size, mass, inertia and aerodynamic behavior; potential electrical spark-overs; and changes to the transmission of light which could lower sensitivity and hence lower reliability when used with gas sensors.
A need thus persists for a highly efficient particle remover which does not appreciably alter the thermodynamic conditions or chemical composition of the gas phase of the aerosol, the function of which is not influenced by the removed particles, and which facilitates quick and easy alternating operation.
The present invention provides apparatus and a method which overcome the deficiencies described above and provide additional significant benefits. Pursuant to the teachings of this invention, particles can be readily and efficiently removed from an aerosol with no attendant pressure drop or temperature change, and no or minimal change to the aerosol""s gas composition.
In accordance with a first general aspect of the invention, apparatus for removing particles from an aerosol is provided. The apparatus includes a particle charger for imparting a charge to particles in an aerosol without affecting thermodynamic characteristics or chemical composition of the gas phase of the aerosol. Charged particles in the aerosol are deflected to provide a portion which is particle free but otherwise substantially identical to the aerosol. This portion is then physically separated from the aerosol. The particle charger may include means for aerodynamically substantially preventing any gas components produced by the particle charger from reaching the aerosol, except for ions to charge the particles.
In a second aspect, a method for removing particles from an aerosol is provided. A charge is imparted to particles in the aerosol; alteration of the chemical composition of the gas phase of the aerosol is prevented. The charged particles are deflected to produce a particle free portion which is separated from the aerosol.
In another aspect, a gas particle partitioner is provided. The partitioner includes a selectively activatable particle charger for producing charged particles in an aerosol with no appreciable change to the chemical composition of the gas phase of the aerosol. A fractionator operates on said charged particles to fractionate the aerosol into a particle laden gas stream and a particle free gas stream. A flow splitter separates said particle free gas stream from the particle laden gas stream.
The particle charger may comprise a corona discharger and a permeable electrode. Ions from the corona discharger are transported through the permeable electrode to interact with and electrically charge particles in the aerosol. The permeable electrode may separate a corona discharge area on one side of the electrode from an aerosol charging zone on another side of the electrode. A particle free fluid may wash the corona discharge area to minimize any transport of gas components produced by corona discharge from said corona discharger to the aerosol. The particle free fluid may comprise an air flow, and means may be provided for regulating the air flow and flow of the aerosol to isokinetic conditions to disallow gas exchange between the air flow and the aerosol.
The corona discharger may comprise a corona discharge wire, made, e.g. of electrically conducting material, preferably silver, switchably connectable to a corona voltage source. A permeable grid electrode may surround the corona discharge wire such that when an additional voltage is applied to the grid electrode, an electric field is produced in the space between the grid electrode and an outer wall, and ions are transported through openings in the electrode due to this electric field.
Further, means may be provided for controlling ion production by the corona discharger in response to a measurement of ionic current produced by the corona discharge. A shielded connector is advantageously employed in the measurement of ionic current.
The gas particle partitioner may also include an aerosol inlet for producing a laminar flow of the aerosol to the particle charger. The fractionator of the gas particle partitioner may include a first electrode, a second electrode spaced from the first electrode, and means for selectively applying an electric field between these electrodes, such that, when an aerosol flows between the first and second electrodes, the charged particles in the aerosol are deflected towards the second electrode by the applied electric field. The fractionator produces a particle free gas stream adjacent the first electrode and a particle laden gas stream adjacent the second electrode when the electric field is applied. The first electrode may comprise an inner cylindrical wall and the second electrode may comprise an outer cylindrical wall. The means for selectively applying an electric field between the first and second electrodes may comprise a voltage supply switchably connectable to at least one of these electrodes, and a shunt resistor for minimizing switching dead time.
The flow splitter of the gas particle partitioner may comprise a conductive ring located near an outlet of the fractionator, and means for applying a voltage to this ring.
The present invention provides numerous significant benefits and advantages. Foremost among these is the ability to separate and remove particles from an aerosol with high efficiency and without altering the thermodynamic conditions and chemical composition of the gas phase of the aerosol. Unlike fabric filters, there is no pressure drop with the present invention which permits the use of smaller pumps and provides lower acquisition and maintenance costs. Since there is no change to the thermodynamic conditions of the aerosol, measures to stabilize such conditions can be avoided. The prevention of changes to the gas composition of the aerosol enables use of the gas particle partitioner (GPP) in gas measuring devices, and reduction of unfavorable gas reactions, corrosion, etc.
Further, in the present invention, the removed particles have no influence on the functionality of the GPP resulting in longer lifetime and cost reduction. The apparatus of the present invention is also easy to switch on and off, enabling studies of particle and gas effects and interactions. An integrated isokinetic flow split avoids changes to the original particle size distribution and concentration for defined conditions. The gas particle partitioner of the present invention also exhibits low energy consumption, good chemical resistance, minimal soiling inside and easy handling. Further, the design is extremely versatile and can be used in a wide variety of applications.